I'm not sure even guaranteeing the AI a KO is enough to deter it from Protecting - I've seen random Protects even when the mon could secure a KO with one of its moves. It already has a 6.3% chance to KO, which seems like it'd be a move the AI likes plenty even without cutting bulk.
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Edit 4: More on Medicham, seems like it can also OHKO Gyarados with critical HJK, so switching in Gyarados on a HJK aimed at Durant is not safe at all. And saccing Durant would be even less safe with Gyarados only being able to get to +1, and needing to Mega Evolve for that, denying the possibility of Intimidate set-up later. I'm thinking of re-EVing Gyarados for extra Defense in order to guarantee survival on critical HJK to solve the Medicham problem for good.
I've found that the AI does seem to go for a KO over Protect, but it may be only over a certain threshold of damage (like, a minimum of 102% may not be enough), or it may need a guaranteed KO. IIRC, everything with Protect attacked Sableye rather than going for Protect, because they could do a million percent damage or whatever. I also found that, against Yanmega 4, switching in Durant to take an attack (and getting it down to 30% health or less) would lead Yanmega to ALWAYS use an attack the next turn (rather than Detect), but it would typically use Detect against a full health Durant (which it can't OHKO without a crit). That quirk was actually the basis of my strategy against possible Speed Boost Yanmega; switching in Durant to take the attack led me to always be able to outspeed and use Entrainment the next turn, while sacrificing Sableye to bring Durant in at full health meant it would use Detect the next turn.
I just tested mock battles against Medicham. Pure Power Medicham seems like it always uses HJK against Durant; I'm pretty sure Telepathy Medicham will use Fake Out or Detect (I only tested one battle against Telepathy Medicham and it used Fake Out). I think using Entrainment against it is fine; if it's Pure Power, you get rid of its ability and make it a weakling (I just saw a crit HJK not KO Durant), and if it's Telepathy, it will block Entrainment but is a weakling anyway, meaning Gyarados doesn't have to worry too much about switching into HJK. And if it's Pure Power, the turns will go Entrainment/HJK (doesn't KO Durant), switch to Gyarados/loafs, Mega Evolve+Sub/Psycho Cut or (more likely) Detect), Dragon Dance/loaf, then you can just Protect on the HJKs and get free Dragon Dances on the loaf turns. For Telepathy, you can go for the +1 or you can switch out to Drapion to take Psycho Cut then back to Gyarados to put it at -2, at which point HJK can't 3HKO Drapion (though with 252 HP/252 Def, Telepathy Medicham won't 3HKO Drapion even at -1; I forget your EV spread for Drapion).